1. Field of the Invention
The present invention relates generally to proteomics and to the analysis of peptides and proteins in simple and complex mixtures. More specifically, the present invention relates to using liquid chromatography in combination with mass spectrometry to produce peptide precursor ions and fragment ions and to group these peptide precursor ions and fragment ions using selected ion chromatograms.
2. Background of the Invention
Proteomics generally refers to studies involving simple and complex mixtures of proteins. Proteomic studies typically focus on identification or cataloging of proteins in biological systems, or determination of changes in relative abundance among different conditions in biological systems, or both. Identification and quantification of proteins in biological samples is a fundamental problem in proteomics.
Identification and quantification of proteins is crucial to understanding and combating disease, to discovering disease biomarkers, to studying metabolic pathways, and to identifying protein targets in drug discovery. A vital tool in proteomic studies, used to identify and to quantify proteins, is liquid chromatography combined with electrospray ionization mass spectrometry (ESI-LC/MS).
In conventional proteomic studies, proteins of interest typically are first digested to produce a specific set of proteolytic peptides rather than studying the intact proteins directly. The resulting peptides are then characterized during the proteomic analysis. A common proteolytic enzyme used for such digestion is trypsin. In tryptic digestion, the proteins present in the complex mixture are cleaved to produce peptides as determined by the trypsin's cleavage specificity. The trypsin enzyme cleaves proteins along the C-terminal side of the amino acids lysine and arginine.
In LC/MS analysis, the peptide digest is separated and analyzed by liquid chromatographic (LC) separation followed by on-line mass spectrometric (MS) analysis. In the LC separation, the interaction of a peptide with the stationary and mobile phases determines the retention time and chromatographic peak shape of that peptide. We use the term originating molecule to refer to the neutral molecule that is separated by the LC. In the case of tryptic digests, the originating molecules are peptides. The output eluent of the LC, containing the separated, originating molecules is passed to the mass spectrometer.
The ionization source in the mass spectrometer ionizes the originating molecules. The ions corresponding to the originating molecules are called precursors. Once introduced into the mass spectrometer, the precursors can be collisionally dissociated or fragmented into fragment ions. It is the precursor ions and the fragment ions from the originating molecules that are analyzed or mass-measured by the mass spectrometer.
A common method for identifying peptides in a mixture is to compare the mass-analyzed ions to a database containing groups of ions corresponding to known peptides. To make the comparison, the mass analyzed ions must be grouped together into groups of related ions. That is, fragment ions are grouped together into groups likely coming from the same precursor ion.
Conventional techniques make such ions grouping using a tandem mass spectrometer and data dependent selection and fragmentation of precursor ions. A tandem mass spectrometer can select precursors in a first mass spectrometer, can collisionally fragment the selected precursors in a collision cell, and analyze the resulting fragments in a second mass spectrometer. In such data dependent analyses (DDA), groupings are made based solely on precursor ion selection in the first mass spectrometer. However, multiple originating molecules can have chromatographic peaks that overlap in time and have m/z values that lie within the transmission window of the first mass spectrometer. In such a case the fragmentation spectrum obtained in the second mass spectrometer will contain fragments from multiple precursors. As a result, conventional techniques can inadvertently group ions that in fact come from two or more distinct originating molecules.
An improved means to group ions is described by U.S. Pat. No. 6,717,130 to Bateman (“Bateman”), hereby incorporated by reference. In Bateman, spectra from precursors and fragments are obtained using a high- and low-energy switching protocol applied as part of an LC/MS analysis of a single injection of a peptide mixture. In such data, the low-energy spectra contain ions primarily from unfragmented precursors, while the high-energy spectra contain ions primarily from fragmented precursors. Thus this protocol collects spectra in two modes, a low-energy mode and a high-energy mode.
The output of this protocol for each mode is a series of mass spectra over time. The mass spectra can be combined in time to produce a set of selected ion chromatograms. Selected ion chromatograms can also be referred to as mass chromatograms. Hereafter, the terms “selected ion chromatogram” and “mass chromatogram” are used interchangeably. The selected ion or mass chromatograms can be searched for chromatographic peaks. The peaks identify likely ions. Each ion is described by its apex retention time, mass-to-charge ratio, and intensity. Co-pending PCT Application No. PCT/US05/04180, filed Feb. 11, 2005, entitled “Apparatus and Method for Identifying Peaks in Liquid Chromatography/Mass Spectrometry Data and for Forming Spectra and Chromatograms,” (the “'4180 application”), hereby incorporated by reference, describes a two-dimensional convolution technique that can be applied to such spectra to obtain lists of ions and each ion's apex retention time, mass-to-charge ratio, and intensity.
Thus, the output of the LC/MS system is an inventory, or list, of precursor and fragment ions, each ion described by their apex retention time, mass-to-charge ratio, and intensity. The low-energy mode produces a list of ion seen in low energy that contains primarily unfragmented precursor ions. The high-energy mode produces a list of ion seen in high energy that contains primarily fragmented precursor ions.
Bateman describes how ion groupings can be made based solely upon retention time of the chromatographic peaks as seen in mass chromatograms. The '4180 application describes how ion groupings can be made based solely upon the retention time of ions.
Two ions that have different retention times must derive from different originating molecules. Thus grouping based on retention time can eliminate ions that elute within a chromatographic peak width of a precursor. By requiring that ions in a group must have the same retention time, the methods of Bateman and the '4180 application will exclude ions that have different retention times even if their peaks overlap chromatographically.
However, in complex mixtures, it may yet be that multiple originating molecules still elute at essentially the same retention time. As a result, even the improved method described in Bateman and the '4180 application, which group ions solely on the basis of retention time, can inadvertently group ions that in fact come from two or more distinct eluting molecules.